1. Field of the Invention
The present invention relates to a microwave plasma processing apparatus and method capable of maintaining a high-density and large-area uniform plasma even in a low-pressure region.
2. Related Background Art
As a plasma processing apparatus using a microwave as an excitation source for producing a plasma, a CVD apparatus, an etching apparatus, or the like is known.
For example, film formation using such a so-called microwave plasma CVD apparatus is performed in the following manner. More specifically, a gas is supplied into a plasma generation chamber and a film formation chamber in the microwave plasma CVD apparatus, and, at the same time, microwave energy is applied to the microwave plasma CVD apparatus to generate a plasma in the plasma generation chamber, thereby exciting and resolving the gas. In this manner, a deposition film is formed on a substrate arranged in the film formation chamber.
An etching or ashing process, using a so-called microwave plasma etching apparatus, for a substrate to be processed is performed as follows. More specifically, a process gas is supplied into a processing chamber of the apparatus, and, at the same time, microwave energy is applied to the apparatus to excite and resolve the process gas, thereby generating a plasma in the processing chamber. In this manner, the surface of a substrate to be processed arranged in the processing chamber is subjected to etching or ashing.
In the microwave plasma processing apparatus, a microwave serving as a gas excitation source is used. For this reason, electrons can be accelerated by an electric field having a high frequency, and gas molecules can be efficiently ionized and excited. Therefore, the microwave plasma processing apparatus has the following advantages. First, gas ionization efficiency, gas excitation efficiency, and gas resolution efficiency are high, a plasma having a high density can be reliably and easily formed, and a high-quality process can be rapidly performed at a low temperature. In addition, since a microwave transmits through a dielectric, a plasma processing apparatus can be constituted as an electrodeless discharge type apparatus. For this reason, plasma processing having high cleanness can be advantageously performed.
In order to further increase the speed of such a microwave plasma processing apparatus, a plasma processing apparatus using electron cyclotron resonance (ECR) has been actually used. ECR is the following phenomenon. When a magnetic flux density is 87.5 mT, an electron cyclotron frequency at which electrons are rotated around a magnetic flux line is equal to the general frequency of a microwave, i.e., 2.45 GHz, the electrons resonantly absorb the microwave and are accelerated, thereby generating a high-density plasma. In the ECR plasma processing apparatus as described above, as an arrangement between a microwave feeding means and a magnetic field generation means, four typical arrangements are known.
More specifically, the following arrangements are used. (i) A microwave transmitted through a waveguide is fed into a cylindrical plasma generation chamber through a transparent window from a surface opposing a substrate to be processed, and a divergent magnetic field having the same axis as that of the central axis of the plasma generation chamber is fed through a single electromagnetic coil arranged around the plasma generation chamber (NTT scheme). (ii) A microwave transmitted through a waveguide is fed into a bell-like plasma generation chamber from a surface opposing a substrate to be processed, and a magnetic field having the same axis as that of the central axis of the plasma generation chamber is fed through a single electromagnetic coil arranged around the plasma generation chamber (HITACHI scheme). (iii) A microwave is fed into a plasma generation chamber through a Lisitano coil, which is a kind of a cylindrical slot antenna, and a magnetic field having the same axis as that of the central axis of the plasma generation chamber is fed through an electromagnetic coil arranged around the plasma generation chamber (Lisitano scheme). (iv) A microwave transmitted through a waveguide is fed into a cylindrical plasma generation chamber through a planar slot antenna from a surface opposing a substrate to be processed, and a loop-like magnetic field parallel to an antenna plane is fed through a permanent magnet arranged on the rear surface of the planar antenna (planar slot antenna scheme).
However, even in the above apparatus, a plasma density near the surface of the substrate to be processed is disadvantageously ununiform. On the other hand, in recent years, an apparatus using a circular waveguide having a plurality of slots formed in the inside surface of the circular waveguide has been proposed as an apparatus for uniformly and efficiently feeding a microwave (Japanese Patent Application No. 3-293010). This microwave plasma processing apparatus is shown in FIG. 5A. Reference numeral 1101 denotes a plasma generation chamber; 1102, a quartz tube constituting the plasma generation chamber 1101; 1103, a circular waveguide with slots for feeding a microwave into the plasma generation chamber 1101; 1104, a plasma generation gas supply means; 1111, a processing chamber communicating with the plasma generation chamber 1101; 1112, a substrate to be processed; 1113, a support member for the substrate 1112; 1114, a heater for heating the substrate 1112; 1115, a process gas supply means; and 1116, an exhaust. FIG. 5B is a sectional view showing the processing apparatus along a line 5B--5B in FIG. 5A. Reference numeral 1120 denotes slots. Plasma generation and plasma processing are performed as follows. The plasma generation chamber 1101 and the processing chamber 1111 are evacuated through an exhaust system (not shown). A plasma generation gas is supplied into the plasma generation chamber 1101 at a predetermined flow rate through the gas supply port 1104. A conductance valve (not shown) arranged in the exhaust system (not shown) is adjusted to hold the plasma generation chamber 1101 at a predetermined pressure. A desired power is supplied from a microwave power supply (not shown) into the plasma generation chamber 1101 through the circular waveguide 1103. Electrons are accelerated by the microwave electric field to produce a high-density plasma in the plasma generation chamber 1101. At this time, when a process gas is supplied into the processing chamber 1111 through the process gas supply means 1115, the process gas is excited by the produced high-density plasma to process the surface of the substrate to be processed 1112 placed on the support member 1113. The process gas may be supplied into the plasma generation gas supply port 1104, depending on the application.
The plasma processing is desirably performed at a temperature which is lowered as much as possible. This is because a plastic substrate in an MCD or a plastic LCD must be prevented from being deformed or resolved. In addition, in a ULSI, diffusion of an impurity, hillocks of Al wiring, thermal distortion, and the like must be prevented.
When the microwave plasma processing apparatus as described above is used, a low-temperature and high-density uniform plasma having an electron temperature of 3 eV or less and an electron density of about 10.sup.12 /cm.sup.2 can be produced in a large-diameter space. A gas sufficiently reacts with the plasma, and the gas can be supplied to the substrate in an active state. For this reason, high-quality and high-speed processing can be performed at a low temperature.
However, when processing is to be performed at a low temperature by using the microwave plasma processing apparatus which a low-temperature and high-density plasma is produced as shown in FIGS. 5A and 5B, there are the following points to be improved. More specifically, in plasma CVD, when a pressure is set to be 1 mTorr or less, the mean free path of precursor atoms increases, and the precursor atoms reach the substrate with sufficient energy. For this reason, film quality is desirably improved. However, electron density decreases in a region having a pressure of 1 mTorr or less, and unstable discharge is unsatisfactorily performed, which is unsatisfactory.
It is a main object of the present invention to provide a microwave plasma processing apparatus and a microwave plasma processing method in which nonununiformity of a plasma near a substrate surface is canceled, and a high-density and large-area uniform plasma can be maintained even at a low temperature and even in a low-pressure region having a pressure of 1 mTorr or less.